Best Underlayment for Outdoor Porcelain Tile Deck

I’m looking to replace a traditional wood deck with porcelain tile (looks like hardwood). What is the best underlayment to use? The patio is mostly covered, but there is still some exposure to the elements (Midwest).

Would a 3/4” plywood base with some weatherproof cover be a proper subfloor? Plywood feels like a bad decision since the underside would be fully exposed.